1. Field of the Invention
The present invention relates to a pulse tube refrigerator driven by an oil free type compressor, and in particular to a compressor integrated pulse tube refrigerator of an oil free type which is capable of maintaining an accurate gap between an inner surface of a cylinder and an outer surface of a piston so that a gas is not leaked through the gap to the outside in a state that the piston does not contact with an inner surface of the cylinder when the piston reciprocates within the cylinder.
2. Description of the Background Art
Generally, as a ultra low temperature refrigerator which is used for cooling a small size electronic component and a super-conductive material, a thermal reproducing type refrigerator such as a Stirling refrigerator, a GM refrigerator, etc. is used.
The resistance of most typical electronic components are decreased at a low temperature for thereby increasing an operational efficiency of the components, and the processing speed of a CPU(Central Processing Unit) used for a computer is increased.
In addition, as the super-conductive product is intensively studied, the need for a low temperature price ultra low refrigerator which is capable of satisfying the cooling conditions of the small size components is gradually increased.
In order to increase the reliability of the above-described refrigerator, the operation speed is decreased, or a lubricating operation is enhanced for preventing an abrasion between the friction portions during a pumping operation of a working gas, or the characteristic of a sealant is improved. In addition, the number of the operational portions is decreased.
Recently, as a ultra low temperature refrigerator which has a high reliable operation and is capable of implementing a high speed operation and does not need an additional lubricating operation and a maintenance for a long time, an oil free type compressor pulse tube refrigerator is disclosed.
The above-described oil free type compressor pulse tube refrigerator is directed to implementing a ultra low temperature refrigerating operation at an open side of the tube using a principle that when varying a pressure by periodically injecting a gas having a certain temperature into a one side-blocked tube, a large temperature variation is obtained at a portion in which there is a turbulent flow of the gas. Namely, the oil free type compressor pulse tube refrigerator is a refrigerator having a low average pressure and pressure ratio and a low refrigerating capacity. In the oil free type compressor pulse tube refrigerator, the pulse tube refrigerator includes one movement unit of a compressor compared to the conventional Stirling refrigerator having two movement units of a piston and displacer.
As a pulse tube refrigerator, there are a basic type pulse tube refrigerator, a resonance type pulse tube refrigerator having an acoustic driving unit, a hole type pulse tube refrigerator fabricated by installing an orifice, which generates a phase difference of a pressure pulse and a mas flow rate, and a storing container at the basic type pulse tube refrigerator, and an inertia tube type pulse tube refrigerator using an inertance tube(long neck tube) instead of the orifice. Among the above-described refrigerators, the basic type pulse tube refrigerator, the hole type pulse tube refrigerator and the inertia tube type pulse tube refrigerator will be explained.
First, as shown in FIG. 1, the basic type pulse tube refrigerator includes a driving unit M, a hollow pulse tube 1 having a warm end 1a and a cold end 1b for introducing a working gas pumped by the driving unit M for thereby compressing and expanding the gas therein, and a reproducing unit 2 connected between the driving unit M and the pulse tube 1 for maintaining a certain temperature of the working gas which contains a sensible heat due to a temperature difference based on the compressing and expanding operations of the working gas.
In the drawing, reference numerals 2a and 2b represent the connection tubes.
The operation of the basic type pulse tube refrigerator will be explained with reference to the accompanying drawings.
First, when the driving unit M pushes the working gas into the interior of the reproducing unit 2, the thusly pushed high temperature and pressure working gas having a sensible heat flows through the reproducing unit 2 and is flown into the pulse tube 1. The working gas in the pulse tube 1 is flown toward the blocked side and then is more compressed. At the warm end portion 1 a, a heat is radiated based on a heat transfer operation at the tube wall.
On the contrary, when the driving unit M sucks the working gas, the gas introduced into the interior of the pulse tube 1 is discharged, and the working gas in the pulse tube 1 is expanded, the heat is absorbed at the cold end 1b by a heat transfer at the tube wall. The above-described operation is repeatedly performed, so that it is possible to obtain a ultra low temperature(about -20.degree. C.) at the cold end. At this time, the working gas discharged from the pulse tube 1 absorbs the heat stored in the reproducing unit 2 and is heated by a certain temperature and is introduced into the driving unit M.
The hole type pulse tube refrigerator will be explained with reference to the accompanying drawing.
First, as shown in FIG. 2, the hole type pulse refrigerator includes a driving unit M, a pulse tube 3 having a warm end portion 3a at which a gas is compressed and a cold end 3b at which a gas is expanded, as the working gas pumped by the driving unit M is inwardly introduced for thereby implementing a certain mass flow rate of the working gas, an orifice 4 connected with the warm end portion 3a of the pulse tube 3 for generating a certain phase difference based on the mass flow rate of the flowing working gas and the pressure pulse operation, a storing container 5 connected with the orifice 4 and holding the working gas therein for a certain time, and a reproducing unit 6 connected between the cold end 3b and the driving unit M for storing a sensible heat of the working gas pumped toward the pulse tube 3 and supplying the stored heat when the working gas flows from the pulse tube 3 to the driving unit M.
In the drawing, reference numerals 4a, 6a and 6b represent the connection tube.
The operation of the hole type pulse tube refrigerator is similar with the basic type pulse tube refrigerator except for the following difference. Namely, in the basic type pulse tube refrigerator, the heat is radiated from the working gas via the tube wall of the pulse tube 1. In the hole type pulse tube refrigerator, the working gas flows through the orifice 4 and increases the phase difference between the mass flow rate and the pressure pulse operation based on an adiabatic expansion for thereby obtaining a higher cooling capability.
Namely, in the hole type pulse tube refrigerator, when the working gas is supplied by the driving unit M and flows via the reproducing unit 6 and is introduced into the pulse tube 3, the working gas filled in the pulse tube 3 is adiabatically compressed, so that the temperature of the working gas is increased and is penetrated into the orifice 4, whereby the working gas is expanded by the orifice 4 and is filled in the storing container 5.
In addition, in the basic pulse tube refrigerator, the working gas is re-heated by receiving the heat from the tube wall, and in the hole type pulse refrigerator, the working gas is heated while the working gas flows the orifice 4 and is adiabatically compressed in the pulse tube 3.
When the working gas is sucked by the driving unit M, the working gas is adiabatically expanded due to a mass flow rate difference between the working gas flown from the pulse tube 3 and the working gas introduced into the pulse tube 3 via the orifice 4 when the working gas is flown from the pulse tube 3 to the reproducing unit 6, so that the temperature of the working gas is decreased.
The working gas in the pulse tube 3 is compressed by the working gas which is continuously introduced via the orifice 4, so that a ultra low temperature refrigerating effect of the pulse tube is obtained by the above-described processes.
In addition, in the inertia tube type pulse tube refrigerator which uses a lengthy tube having a small diameter instead of the orifice, it is possible to enhance the performance by increasing the variation of the phase difference between the mass flow rate and the pressure pulse operation.
The above-described pulse tube refrigerator and the inertia tube type pulse tube refrigerator generate a higher refrigerating capability based on the phase difference between the mass flow rate and the pressure pulse differently from the basic type refrigerator. The orifice and inertia tube are called as a phase controller(or a phase device or a phase developer). The hole type and inertia type pulse refrigerator(hereinafter called as a "Pulse tube refrigerator") will be explained.
As shown in FIG. 3, the conventional pulse tube refrigerator includes a driving unit 10 for generating a reciprocating flow of the working gas, a refrigerating unit 20 for having a ultra low temperature portion based on a thermal mechanics cycling operation of the working gas which reciprocates in the tube by the driving unit 10, and a valve selectively communicating the driving unit 10 and the refrigerating unit 20.
The structures of the driving unit 10 and the refrigerating unit 20 will be explained in detail.
The driving unit 10 includes a compressor 11 used for a common refrigerator using a lubricating oil, a low pressure container 12 installed at an inlet of the compressor 11 for storing a low pressure suction gas, a high pressure container 13 installed at an outlet of the compressor 11 for storing a high pressure exhausting gas, and an oil separating unit 14 installed between the high pressure container 13 and the outlet of the compressor 11 for removing an oil contained in the working gas and supplying the working gas to the compressor 11.
In the drawings, reference numerals 11a, 11b, 11c, 12a, 13a, and 14a represent the connection tubes.
The refrigerating unit 20 includes a pulse tube 21 having a compression portion 21a at which a compression is performed for thereby generating a heat and an expansion portion 21b at which an expansion is performed for thereby absorbing a heat as the working gas is mass-flown and a compression and expansion are performed at both ends of the same by the working gas pumped by the driving unit 10, an orifice 22 connected with the compression unit 21a of the pulse tube 21 for generating a phase difference between the mass flow rate of the working gas and the pressure pulse and implementing a thermal balance state, a storing container 23 connected with the orifice 22 for temporarily storing the working gas, a reproducing unit 24 connected between the expansion unit 21b of the pulse tube 21 and the driving unit 10 for compensating the temperature of the working gas returning from the pulse tube 21 to the driving unit, and a pre-cooling unit 25 connected between the reproducing unit 24 and the driving unit 10 for pre-cooling a high temperature and pressure working gas pumped from the driving unit 10.
The valve 30 is a rotary valve for repeatedly communicating the low pressure container 12 and the pre-cooling unit 25 or the high pressure container 13 and the pre-cooler 25 at a certain time interval and is installed between the low pressure container 12 and the high pressure container 13 of the driving unit 10 and the pre-cooling unit 25 of the refrigerating unit 20.
In the drawings, reference numeral 15 represents a driving unit casing, and 30a and 22a represent the connection tubes.
The operation of the conventional pulse tube refrigerator will be explained with reference to the accompanying drawings.
First, a low temperature and pressure working gas charged in the low pressure container 12 is compressed and changed to a high temperature and pressure working gas by the compressor 11 and passes trough the oil separating unit 14 and is stored in the high pressure container 13.
At this time, the oil separating unit 14 separates the oil contained in the working gas and outputs the separated oil to the compressor 11 and outputs the gas to the high pressure container 13.
First, the valve 30 communicates the high pressure container 13 and the refrigerating unit 20, and a high pressure working gas is cooled by the pre-cooling unit 25 and the reproducing unit 24 and is flown into the pulse tube 21. The working gas introduced into the pulse tube 21 pushes the working gas filled in the pulse tube 21 toward the orifice 22. At this time, the working gas filled in the pulse tube 21 is in a thermal balance state with respect to the tube wall and is moved toward the orifice 22, so that the working gas is adiabatically compressed, and the temperature of the same is increased.
As the valve 30 is closed, the pressure in the pulse tube 21 is maintained in a high pressure state, and the working gas in the pulse tube 21 is flown toward the lower pressure side storing container 23 via the orifice 22. During the above-described operation, the working gas is adiabatically expanded for thereby radiating the heat to the outside. The working gas in the pulse tube 21 becomes a thermal balance state at a temperature lower than at the initial state of the operation.
Thereafter, when the valve 30 communicates the low pressure container 13 and the refrigerating unit 10, the low temperature working gas filled in the pulse tube 21 is moved toward the low pressure container 12. The working gas moved toward the storing container 23 is moved again toward the pulse tube 21. At this time, the mass flow rate of the working gas which is flown from the pulse tube 21 via the reproducing unit 24 is greater than the mass flow rate of the working gas introduced into the pulse tube 21 via the orifice 22. Therefore, the working gas in the expansion unit 21b of the pulse tube 21 is rapidly adiabatically expanded, and the temperature of the same becomes a ultra low temperature.
Next, the valve 30 is closed. When the pressure in the pulse tube 321 is low, the working gas is flown into the pulse tube 21 from the storing container 23 to the orifice 22, so that the working gas in the pulse tube 21 is compressed, and the temperature of the same is increased up to the temperature before the driving operation. The above-described operation forms one cycle.
The working gas introduced into the low pressure container 12 via the reproducing unit 24 and the pre-cooling unit 25 is flown into the compressor 11 and is compressed therein. The thusly compressed working gas is filled into the high pressure container 13. When the valve 30 is opened, the working gas is flown again into the pulse tube 21. The above-described cycle is repeatedly performed. The temperature of the expansion unit 21b of the pulse tube 21 is decreased to about -200.degree. C.
However, in the conventional pulse tube refrigerator, the structure of the refrigerator is simple. However, the driving unit includes a compressor, high/low pressure containers, an oil separating unit, etc. Therefore, the size of the system is too large. Since the elements such as the compressor, the high and low pressure container, the oil separating unit, etc. are independently assembled for forming one driving unit, the number of the assembling processes is increased, and the assembling time is extended.
In addition, due to a limitation with respect to the operation speed of the valve which selectively connects the driving unit and the refrigerating unit, it is impossible to properly supply a working gas to the refrigerating unit. The working gas which passes through the valve is adiabatically expanded, so that the efficiency of the refrigerator is decreased.